Dust cores to be used in an AC magnetic field are required to have low core loss and a high magnetic flux density. Moreover, they are also required to have good mechanical properties in the manufacturing process and high resistance to damage at the time of coil winding. One technology has been developed to meet these requirements for dust cores by coating iron powder particles with a synthetic resin. The coating film of electrically insulating synthetic resin suppresses eddy current loss and bonds iron powder particles together, thereby improving their mechanical strength.
Dust cores have recently come into general use as the core for motors. Conventional cores for motors were magnetic steel sheets or electric steel sheets in laminate form. Unlike such conventional cores, dust cores are easily produced in any three-dimensional shape by compression molding. Therefore, motors with dust cores are smaller and lighter than conventional ones. The dust cores for small and light motors are required to have a higher magnetic flux density, lower core loss, and higher mechanical strength than before.
Increase in magnetic flux density will be effectively achieved by densely forming the compact. Decrease in core loss (particularly hysteresis loss) will be effectively achieved by annealing at high temperatures to relieve strains of the compacts. The foregoing has stimulated the development of a new iron powder for dust cores which keeps electric isolation of iron powder particles even though the amount of the insulating material is reduced for dense forming and which also keeps good electrical insulation even after heat treatment (such as annealing) at high temperatures.
From the foregoing view point, there has been developed a technology to use a highly heat-resistant silicone resin as the insulating material. For example, the technology disclosed in Patent Document 1 employs a specific methyl-phenyl silicone resin as the insulating material. However, it needs not less than 1 mass % of silicone resin (based on iron powder) for good thermal stability and hence it has room for improvement in high-density forming. There are other technologies to ensure heat resistance by incorporation of silicone resin with glass powder or pigment as disclosed in Patent Documents 2 and 3. Their disadvantage is that incorporation with glass powder or pigment is undesirable for density increase.
There is another technology that employs a coating film of glassy compound (derived from phosphoric acid) as the insulating material other than synthetic resin, as disclosed in Patent Document 4. The inorganic insulating film should be superior in thermal stability to a silicone resin, which is an organic polymer; however, the present inventors found that it becomes poor in insulating performance upon heat treatment (annealing) at high temperatures (as mentioned later).    Patent Document 1.
Japanese Patent Laid-open No. 2002-83709    Patent Document 2.
Japanese Patent Laid-open No. 2004-143554    Patent Document 3.
Japanese Patent Laid-open No. 2003-303711    Patent Document 4.
Japanese Patent No. 2710152